Generally, non-volatile memory devices may be classified as either floating-gate type memory devices or charge-trap type memory devices in accordance with a structure of a unit cell incorporated therein.
The unit cell of a typical floating-gate type non-volatile memory device includes a tunnel oxide layer, a floating gate, a dielectric layer and a control gate sequentially formed on a semiconductor substrate. The floating-gate type non-volatile memory device may store information by charging/discharging electrons into/out of the floating gate.
In contrast, the unit cell of a typical charge-trap type non-volatile memory device includes a tunnel insulation layer including silicon oxide, a charge-trapping layer such as silicon nitride, a blocking layer such as silicon oxide, and an electrode such as doped polysilicon sequentially formed on a semiconductor substrate. In the typical charge-trap type non-volatile memory device, information may be programmed in the charge-trap type non-volatile memory device by storing charges in the charge-trapping layer between the electrode and the semiconductor substrate. Information may be erased from the charge-trap type non-volatile memory device by discharging charges from the charge-trapping layer.
Recently, resistive memory devices with high capacity, capable of operating at a relatively low voltage have been developed. Resistive memory devices are also referred to as “resistive RAMS” or “RRAMS”. The RRAM takes advantage of controllable resistance changes in thin films of variable resistance material, such as thin binary metal oxide films. Integrated circuit resistive memory devices are described, for example, in U.S. Pat. No. 6,849,891 to Hsu et al., and U.S. Pat. No. 7,282,759 to Kim et al.
The resistance node of a resistive non-volatile memory device may vary from a higher resistance state to a lower resistance state according to a voltage applied to both terminals of the resistance node. A conductive path in the shape of a filament may be form on the resistance node, and a current flowing through the resistance node may flow via the conductive path in the shape of the filament. Unfortunately, however, the resistance nodes may have different resistances. Further, performing several processes for changing the resistance at the resistance node may deteriorate reproducibility of manufacturing the non-volatile memory device.